1. Field of the Invention
The present invention is related to specialized metallurgical processes wherein a powder is prepared by the decomposition of an organo-metallic compound in a solution from which the free metal settles.
2. Description of the Prior Art
Fine aluminum powders, for the purposes of the present application, are defined as having particle sizes substantially less than the about 3000-200,000 nm currently available in quantity and obtained by grinding or by spraying into an inert atmosphere. These currently available particles are also of greatly varying size. For example, examination by scanning electron microscopy (SEM) has shown nominal 3000 and 5000 nm powders produced by such spraying to have sizes ranging, respectively, from 200-8500 nm and from 200-11000 nm.
The fine aluminum powders are believed to increase the effectiveness of fuels and fuel additives, pyrotechnics, and energetic materials including composites, thermite, and explosives by a factor of three to ten, this increase being due to the more rapid and complete reaction of the finer particles.
However, this advantage is only practically obtainable if the fine powders can be produced in relatively large quantity and in pre-determined, uniform sizes selectable for the particular use. It is desirable that a method for producing such fine powders in quantity not require expensive equipment, use readily obtainable pressures and temperatures, use relatively inexpensive and non-toxic materials, and provide convenient separation of the product in storable form. Since fine powders of pure aluminum are pyrophoric, it is highly desirable that a practical method for producing such powders provide them in a form that is passivated and yet contains a large amount of pure aluminum.
Insofar as known to the present applicants, there has heretofore been no method that is in accordance with the above listed requirements and advantages and that produces aluminum powders in quantity and with particles of uniform and selectable sizes from 65-500 nm.
Fine aluminum powders have been prepared by exploding aluminum wire in a vacuum by a high electric current; a method requiring expensive equipment. This method provides little or no control of particle size or uniformity, and transmission electron microscopy of its product has shown particles ranging from 50 to 1000 nm in diameter. Very fine aluminum powders have also been prepared by condensation of vaporized aluminum in a current of cold, inert gas; however, relatively high temperatures are required to vaporize the aluminum; expensive equipment is required; and production is relatively slow.
Other metals have been prepared in powder form by decomposition of the carbonyl and by reduction of metal halides in solution. However and insofar as known to applicants, there are no aluminum carbonyls and it is relatively difficult to separate metal powders from the salt solution resulting from such halide reduction.
It is known to plate aluminum on a substrate by the decomposition of a tertiary amine complex of aluminum hydride in vapor form at pressures of up to 30 mm of mercury without a catalyst and at temperatures of 125.degree. to 550.degree. C. Chemical vapor deposition has also been used to plate aluminum from alane adducts on bulk titanium and on silicon. With silicon, (Me.sub.3 N).sub.2 AlH.sub.3 vapor was used at about 0.2 Torr after treatment with TiCl.sub.4 vapor to improve film uniformity and provide average film grain sizes of 1000 nm at 180.degree. C. and 150 nm at 100.degree. C.
U.S. Pat. No. 3,462,288, which issued 19 Aug. 1969, discloses plating aluminum on a substrate from an alkyl or aryl substituted aluminum hydride complexed with an ether or a nitrogen containing compound and catalysed by a compound of "the metals occurring in Groups IVa or Vb of the Periodic Table". It is suggested that the aluminum hydride be employed in solvated form, not only by oxygen or nitrogen containing compounds, but by sulfur or phosphorus containing compounds. It is not mentioned that these latter compounds which, together with arsenic compounds which may also be effective, are typically highly toxic. In one example, the substrate was immersed in a diethyl ether solution of the catalyst, dried at 100.degree. C., immersed in a solution of aluminum hydride in diethyl ether, and again dried at room temperature with an aluminum coating forming in a few minutes where the substrate was contacted by the catalyst solution. In other examples, deposition of the aluminum plate did not occur on a substrate treated with the substituted aluminum hydride and catalyst until initiated by energy in the form of heat, actinic light, or high energy radiation.
It is apparent that generating a powder having uniform particles of a predetermined size from plating or a film on a substrate presents at least as many problems as generating such a powder from bulk metal.
U.S. Pat. No. 3,535,108, which issued 20 Oct. 1970, and 3,578,436, which issued 11 May 1971 to the same inventors, disclose methods for producing purified aluminum in particulate form by the conversion of "crude" aluminum to a dialkylaluminum hydride followed by decomposition of the dialkylaluminum hydride at room to 260.degree. C. temperatures into the purified aluminum together with the corresponding trialkylaluminum and hydrogen which are recycled to convert further crude aluminum into the dialkylaluminum hydride. The reaction system was, apparently, thought to require a tertiary amine as well as a catalyst including at least one compound of titanium, zirconium, hafnium, vanadium, a lanthanide, or an actinide in an weight ratio of 0.01 through 0.00001 to the produced aluminum. However, it was discovered that the tertiary amine need not be present. Evidently, the size and uniformity of the aluminum particles was uncontrolled except that it was thought advantageous to increase the average size of the particles by seeding the system with "finely divided aluminum powder".
In a related method, decomposition of diethylhydridoaluminum or diisobutylhydridoaluminum in diisopropyl ether or triethylamine at 90.degree. to 185.degree. C., produced at least 99.97 percent pure particulate aluminum along with twice the molar quantity of the corresponding trialkylaluminum. Titanium isopropoxide catalyst was used in an amount by weight of 1 part per 3000 parts aluminum produced, and the particles were nonpyrophoric conglomerates of 500,000 nm. These conglomerates were reducible by "intense grinding" to 420 nm mean particle diameter based on surface area measurement.